32
3
in cAMP on isolated particles from mammalian myocardium, liver or cerebral cortex after
stimulation with catecholamines (epinephrine, also called adrenaline and norepinephrine, also called noradrenaline – these names indicate that these compounds are released
from the adrenal gland). In this way, they established a link between hormone action and
the enzyme adenylyl cyclase, which produces cAMP from ATP. They inferred that adenylyl cyclase was an allosterically regulated enzyme with a regulatory site somehow responsive to catecholamines and asked whether adenylyl cyclase was the membrane receptor for
catecholamines (see 7 Box 3.1).
This was considered unlikely after experimental results showing that molecules with
vastly different structures could induce cAMP release from fat cell membranes. The tested
molecules included the catecholamine adrenaline and very different peptide hormones,
such as ACTH (adrenocorticotropic hormone), glucagon and TSH (thyroid-stimulating
hormone). It was rather hypothesized that multiple receptors with specificity for their
ligands interacted with a common catalytic unit and that this happened at the cell membrane. Thus, the search for a “transducer” was on. This molecule was supposed to couple
information from outside the cell via a ligand-bound receptor with the regulation of adenylyl cyclase (Rodbell et al. 1968). In 1971, Rodbell proposed a GTP-regulated protein to
be the transducer. The first α-subunit of trimeric G-proteins, which turned out to act as
transducer, was isolated in 1980 in the lab of Alfred Gilman (Northup et al. 1980).
Purification of the α- and β-adrenergic receptors followed some years later (Caron
et al. 1979). In 1986, the genes encoding the α 2 - and β 2 -adrenoreceptors were cloned
(Dixon et al. 1986). Only then it became clear that they were seven-transmembrane receptors. Further work revealed that they were members of a huge family of GPCRs. In all
seven-transmembrane receptors, the N-terminal protein sequences point to the cell exterior; the C-terminal sequences are directed towards the cytoplasm. The seven transmembrane domains are quite conserved between different members of the receptor family. In
contrast, the extracellular and intracellular loops are diverse. The extracellular domains
are involved in ligand binding, and the intracellular domains are responsible for signal
transduction.
3.2 Trimeric G-Proteins
The common “transducers” for all GPCRs are trimeric G-proteins, or GTPases, consisting of
α-, β- and γ-subunits. The α-subunit is structurally related to small GTPases or G- proteins
such as Ras, Ran and Rab (see 7 Box 3.2). These G-proteins alter between a GTP-bound
and a GDP-bound state. In this way, they work as molecular switches. The critical movement of the switch corresponds to the conformational transformation associated with GTP
binding and release. Although the cytoplasm contains much higher concentrations of GTP
Box 3.1 Catecholamine
Catecholamine: compound possessing a dihydrobenzol ring (or catechol ring) and an amino
group. They are also called brenzcatecholamines. Adrenaline, noradrenaline and dopamine are
naturally occurring catecholamines. Synthetic catecholamines include isoprenaline and dobutamine.
Chapter 3 · GPCRs
3
in cAMP on isolated particles from mammalian myocardium, liver or cerebral cortex after
stimulation with catecholamines (epinephrine, also called adrenaline and norepinephrine, also called noradrenaline – these names indicate that these compounds are released
from the adrenal gland). In this way, they established a link between hormone action and
the enzyme adenylyl cyclase, which produces cAMP from ATP. They inferred that adenylyl cyclase was an allosterically regulated enzyme with a regulatory site somehow responsive to catecholamines and asked whether adenylyl cyclase was the membrane receptor for
catecholamines (see 7 Box 3.1).
This was considered unlikely after experimental results showing that molecules with
vastly different structures could induce cAMP release from fat cell membranes. The tested
molecules included the catecholamine adrenaline and very different peptide hormones,
such as ACTH (adrenocorticotropic hormone), glucagon and TSH (thyroid-stimulating
hormone). It was rather hypothesized that multiple receptors with specificity for their
ligands interacted with a common catalytic unit and that this happened at the cell membrane. Thus, the search for a “transducer” was on. This molecule was supposed to couple
information from outside the cell via a ligand-bound receptor with the regulation of adenylyl cyclase (Rodbell et al. 1968). In 1971, Rodbell proposed a GTP-regulated protein to
be the transducer. The first α-subunit of trimeric G-proteins, which turned out to act as
transducer, was isolated in 1980 in the lab of Alfred Gilman (Northup et al. 1980).
Purification of the α- and β-adrenergic receptors followed some years later (Caron
et al. 1979). In 1986, the genes encoding the α 2 - and β 2 -adrenoreceptors were cloned
(Dixon et al. 1986). Only then it became clear that they were seven-transmembrane receptors. Further work revealed that they were members of a huge family of GPCRs. In all
seven-transmembrane receptors, the N-terminal protein sequences point to the cell exterior; the C-terminal sequences are directed towards the cytoplasm. The seven transmembrane domains are quite conserved between different members of the receptor family. In
contrast, the extracellular and intracellular loops are diverse. The extracellular domains
are involved in ligand binding, and the intracellular domains are responsible for signal
transduction.
3.2 Trimeric G-Proteins
The common “transducers” for all GPCRs are trimeric G-proteins, or GTPases, consisting of
α-, β- and γ-subunits. The α-subunit is structurally related to small GTPases or G- proteins
such as Ras, Ran and Rab (see 7 Box 3.2). These G-proteins alter between a GTP-bound
and a GDP-bound state. In this way, they work as molecular switches. The critical movement of the switch corresponds to the conformational transformation associated with GTP
binding and release. Although the cytoplasm contains much higher concentrations of GTP
Box 3.1 Catecholamine
Catecholamine: compound possessing a dihydrobenzol ring (or catechol ring) and an amino
group. They are also called brenzcatecholamines. Adrenaline, noradrenaline and dopamine are
naturally occurring catecholamines. Synthetic catecholamines include isoprenaline and dobutamine.
Chapter 3 · GPCRs